Beam-limiting diaphragm for x-ray tubes



Jan. 26, 1954 K. OSWALD BEAM-LIMITING DIAPHRAGM FOR X-RAY TUBES FiledMarch 4, 1952 INVENTOR Karl swo d By Agent Patented Jan. 26, 1954 UNITEDSTATES PATENT OFFICE BEAM-LIIHIT-ING DIAPHRAGM FOR X-RAY TUBES KarlOswald, 'Eindhoven, Hartford National Ba Hartford, Conn., as trFrequently only part of the X-rays emerging from the window provided inthe sheath of an X-ray tube is used efiectively. In these cases it isadvantageous to restrict the sectional area of the beam with the use ofa diaphragm having a variable aperture.

Use is preferably made of two or more diaphragms, which are difierentlyspaced from the focus of the tube. Constructions are known whichcomprise a driving mechanism, with the use of which the apertures of alldiaphragms are varied simultaneously, When the angle of divergence ofthe emerging X-ray beam must be varied.

For controlling the aperature of so-called sliding diaphragms comprisingtwo pairs of parallel plates, the plates of each pair of plates aredisplaced in opposite sense in a plane at right angles to the beam axis.For the simultaneous control of diaphragms arranged one after the other,the corresponding plates have been connected by rods which are rotatableabout points in the plane through the focus of the tube, at right anglesto the axis of the emerging beam and about two axes intersecting oneanother at right angles. of these axes coincides with the axis of theX-ray tube and. in that the points of rotation are located inside thetube. From a structural point of view this limitation can be obviated,it is true, but this involves complications and enlarges the size of thedevice. In a known embodiment use is made of circular guide members toprovide a correct guidance for the rods.

In a further known construction, the larger aperture is limited byplates rotatable about axes intersecting with one another at rightangles in a plane outside the tube in front of the exit window in thetube sheath. The smaller aperture is limited by a second set of platesrotatable about points in a plane farther away from the focus of thetube. They are moved simultana ously with the first-mentioned plates.

The plates limiting the smaller aperture must be preferably arranged ata small distance from the focus of the X-ray tube. The invention relatesto the construction of a limiting diaphragm having at least twosuccessive apertures, in which the said distance can be minimized.

According to the present invention, the plates limiting the largeraperture are coupled each with a plate of the set of plates limiting thesmaller aperture in a manner such that at a A difiiculty arises hereinthat one Netherlands, assignor to nk and Trust Company,

ustee Application March 4, 1952,, Serial No. 274,765

Claims priority, application Netherlands March 10, 1951 2 Claims. (01.250 -105) variation of the apertures the coupled plates are shifted inopposite sense relative to the beam axis. .For this purpose one plate ofthe pairs of plates bounding the smaller aperture may be linked by acoupling rod to a plate of the set lim iting the larger aperture,located on the other side of the beam axis and the coupling rod may berotatable about points in a plane intersecting with the beam axis atright angles at a point located between the two said platesystems.

In order that the invention may be readily can ried into eifect, oneembodiment thereof will now be described in detail with reference to theaccompanying drawings, in which:

Fig. 1 shows diagrammatically the course or the X-rays and thearrangement of the limiting plates and Figs. 2 and 3 show in two views asimple construction according to the invention.

Fig. 1 shows the anode l of an X-ray tube, viewed in the direction ofthe tube axis" from the cathode. A part 2 of the anode surface isintended to be struck by electrons and is termed the focus of the X-raytube. It constitutes the source of X-rays. This is not the only part ofthe anode surface which is struck by electrons. Secondary and reflectedelectrons strike also the further surface and produce the so-calledscattered radiation. Substantially the entire front of the anodeparticipates in this radiation and although these rays are lesspenetrating than those produced in the focus and although theirinternsity is smaller, the luminescent material of an image surfaceintended to convert the image produced by the X-rays into visibleradiation is ac tivated in a manner such that this scattered radiationproduces a visible luminescence.

The figure shows two sets of closing plates 3 and 4. The set of plates 3is located at asmall distance from the anode and the distance of thesecond set 4 from the anode is considerably larger. The aperturesbetween the plates are chosen such that they exactly permit theeffective X-ray beam to pass.

If onl the plates 3 are provided, not only the efiective ray beam butalso scattered rays emerge" from the aperture, forming a wide beam whichis limited as indicated by the broken lines 5. This beam may cover theentire image surface and continue to cause troublesome additional lightaround the detail to be observed, even if the effective beam of rays isfurther limited.

This undesirable radiation has considerably less effect if only theplates 4 are used. The beam width of the unwanted radiation, indicatedby the broken lines 6, differs in this case little from the width of thebeam of effective X- rays between the full lines 7. Now only at the edgeof the image produced on the image surface a certain amountof additionallight is produced over a small width.

Furthermore the geometric unsharpness produced by the finite dimensionsof the focus from which the X-rays emerge must be considered. The anglebetween two rays coinciding at point 8 and emerging from the edgesofrthe focus 2 is a measure for the unsharpness produced by theeffective X-rays. The unsharpness produced by the scattered radiation isdetermined by the angle under which the rays from the periphery of theanode surface coincide at point 8. If the diaphragm is near the focus,the influence of the last-mentioned radiation is least troublesome. Thisinfluence increases proportionally to the increase in distance betweenthe diaphragm and the focus. Since the detail sharpness of the image isstrongly reduced at an increase in geometrical unsharpness, a diaphragmwill preferably be arranged as near as possible to the :focus. Theseconsiderations lead to the arrange- .ment of two sets of closing platesat different distances from the focus.

Referring to Figs. 2 and 3, such a device is secured to the sheath 9-ofthe X-ray tube 10. The metal wall of the X-ray tube is provided with awindow H to allow the X-rays to pass, which are produced in the focus 2of the anode i. The window H maybe made of glass or of a metal having alow atom numeral, for example, beryllium. The sheath 9 has an aperturein which a piece 13 is fitted. The shape of this piece is chosen suchthat it intimately engages the tube wall. It furthermore closes theaperture in the sheath 9, so that the latter may be filled out with aliquid or gaseous insulating'substance to increase the break-downvoltage and to withdraw heat developed in the anode.

' The closing plates 14 and i5 limit the X-ray beam. They are made ofmaterial, for example, lead, which absorbs X-rays. The plates M areconnected by way of rods it with plates [5,- these rods being rotatablysecured at IT to a ring l2. The latter is connected with the fittingpiece 13, so that the device for limiting the X-ray beam maybe removedwithout detaching the fitting piece I3. If the distance between theclosing plates i5 is decreased or increased, the distance between thclosing plates 14 is also decreased or increased, so that by moving oneset of plates the other set of plates varies the beam width to a 5-corresponding extent.

The device may be extended by one or more sets 'of plates, which arearranged between the said two sets of plates. They may also be displacedby means of the crossing rods I6. They may, for example, serve toprevent secondary X- rays emerging from the edges of the plates 14 fromreaching the image surface.

The connection of the plates with the crossing rods 16 is constituted bytwo studs protruding at right angles to the direction of displacement,on either side of the plates [8 and extending in slots IQ of the rods[6. The studs 18 must be displaceable in the slots 19, since at avariation of the aperture size the distances from the rotary points ofthe rods vary.

When arranging the moving parts, it must be considered that themovability of the inner plate pairs is not hampered by the rodsconnecting the outer plate pairs. For this reason the plates of thesetwo pairs are provided with long studs 18, so that the rods co-operatingherewith may be arranged outside the range covered by the inner plates.

What I claim is:

1. A limiting diaphragm for the X-ray beam of an X-ray tube, comprisinga first two pairs of displaceable radiation-opaque plates disposedadjacent said tube and defining a small aper ture, a second two pairs ofdisplaceable radiation-opaque plates disposed on the side of said firsttwo pairs remote from said tube and defining a larger aperture than saidsmall aperture, and a plurality of coupling rods each interconnecting aplate of said first two pairs located on one side of said Y-ray beam toa plate of said second two pairs located on the other side of said X-raybeam, said coupling rods being pivotable about points lying in a planeat right angles to the axis of the X-ray beam and disposed between saidfirst and second two pairs of plates, whereby the interconnected platesmove in opposite senses relative to the beam axis when the size of theapertures is varied.

2. A limiting diaphragmfor the X-ray beam of an'X-ray tube, comprising afirst two pairs of displaceable radiation-opaque plates disposedadjacent said tube and defining a small aperture, a second two pairs ofdisplaceable radiation-opaque plates disposed on the side ofsaid firsttwo pairs remote from said tube and defining a larger. aperture thansaid small aperture, a plurality of coupling rods each-interconnecting aplate of said first two pairs located on one side. of said X-ray beam toa plate of said second two pairs located on the other sideof said X-raybeam, and a ring-like member disposed between said first two pairs ofplates and said second two pairs of plates and lying in a plane at rightangles to the axis of the X-ray beam, each of said coupling rods beingpivotable about points on said ring-like member, the coupling rodsconnected to the plates of each pair of plates being pivotable about thesame point on the ring-like member, whereby the interconnected platesmove in opposite senses relative to the beam axis when the size of theapertures is varied.

v KARL OSWALD.

References Cited in the file of this patent UNITED STATES PATENTS HauptFeb. 20, 1951

